The need has often arisen in phase locked loops, radar systems such as fast geometric tracking radars, vehicular collision avoidance systems and other such systems seeking, tracking, or measuring velocity of targets, missiles, or projectiles and the like, for deriving indications of the difference in frequency and/or phase between two incoming signals.
Several analog and digital techniques are known in the art for achieving this objective, however they each suffer from a variety of deficiencies. For example, the analog nature of the inputs and outputs of analog frequency and phase discriminators is not particularly convenient for use in digital systems.
This has given rise to various digital implementations using combinations of discrete integrated circuit packages and has even resulted in manufacturers making available self-contained digital frequency and phase detectors in single packages whereby incoming digital pulse trains of unknown frequency or phase may be accepted for processing. However, even with respect to such discrete integrated circuit approaches to frequency and phase detector components, the outputs thereof typically are not easily converted to a conveniently usable digital form.
Moreover, with respect to both prior analog and digital systems, difficulties are further experienced in obtaining frequency or phase difference output indications as well as the tracking by such outputs of one of the input frequencies when decreased from a frequency greater than that of the reference frequency toward the reference frequency.
With respect to the aforementioned prior radar systems, several other problems have been associated therewith. They frequently include a scanning means which typically has a mechanical scanning drive motor associated with a single radar for beam scanning the target. Measurements of the target position are thus derived from processing the scan angle data.
Thus, these systems, particularly those employing mechanical scanning, typically have problems associated with developing scan and scan angle data processing mechanisms which respond rapidly and accurately to fast moving targets while nevertheless being of reasonable cost. This latter factor alone is critical, for example, in providing a collision avoidance system appropriate for the mass consumer automobile market.
Moreover, the expense, response, and accuracy problems become exacerbated when it is necessary to provide a system for detecting vertical as well as horizontal position or motion, requiring yet additional scan and processing mechanisms for the vertical plane. Still further, prior scanning systems have encountered difficulties when employed with large targets or at close ranges (such as those practical for collision avoidance systems) due to the inordinately large scan angles associated therewith.
Accordingly, in summary, a radar system was sought having improved features relative to accuracy, response time, expense, and applicability to close range targets which might otherwise require relatively large scan angles.
More particularly, an improved system was also highly sought after for precisely and quickly providing indications of frequency and/or phase differences between two incoming signals. Methods and apparatus were desirable which provided such indications virtually in real time and in inherently digital form, whereby the pulse rate of the digital output was functionally related to the frequency or phase difference between the two incoming input signals. Such methods and apparatus were further sought which could operate when incoming signals exceeded and approached the reference frequency normally associated with and provided in such frequency/phase difference processing systems.
The disadvantages of the prior art are overcome by the present invention which will be described with reference to the accompanying drawings.